They will be seven coaches long, providing more seating than the three or six-carriage diesel trains they will replace.

ScotRail said the trains would cut journey times because of the electric trains' superior acceleration.

It is understood this could reduce a few off-peak Edinburgh-Glasgow journeys by up to six minutes, with the fastest times around 44 minutes.

ScotRail said they would operate more than one in four services on the line.

The firm said the trains were spares for those which currently operate on routes such as Edinburgh-North Berwick and Dunbar, and Glasgow to Ayr and Gourock.

They should have started running on the main Edinburgh-Glasgow line last December, but the electrification project has been delayed several times.

Problems have included with faulty overhead wire equipment and production difficulties at Hitachi's factory in County Durham.

ScotRail Alliance managing director Alex Hynes has yet to commit to a date for a full electric service on the line, which is due to cut journey times by ten minutes to 42 minutes in December next year.

He told MSPs last month: "I'm not yet prepared to make a commitment [over introduction] because of the complexity of what we are trying to achieve."

Mr Hynes said today: “The running of these electric trains from our existing fleet is an important milestone ahead of the arrival of the brand-new Hitachi trains."

Transport minister Humza Yousaf, who has repeatedly voiced his frustration at the delays in the past, said: “I’d like to offer my personal thanks to all the many people who have worked on this significant electrification project as their efforts come to fruition.”